1. Field of the Invention
This invention relates in general to transducer electrodes for filters or delay lines utilizing surface wave phenomena on piezoelectric substrates wherein the transducers are formed by interdigital fingers with lengths which vary in a predetermined manner.
2. Description of the Prior Art
It is known that metalized surfaces on piezoelectric substrates reduce the propagation velocity of elastic surface waves. In the case of an interdigital transducer for elastic surface waves with longitudinally weighted electrode fingers, for example, this effect causes portions of the wave front in the electrode finger overlap zone to pass through a comparatively dense metalized zone while other portions of the wave front outside the finger overlap zone pass through regions wherein the metallization is relatively small. This causes the wave components which pass through the small metallized transducer zone to lead the wave components passing through the relatively dense metallized zones and causes undesired distortion of the wave front and, thus, discrepancies between the experimental frequency response and the desired frequency response of the transmission element.
So as to prevent this leading effect due to the portion of the wave passing through the smaller metallization zone, it has been known to provide so-called dummy fingers outside the finger electrode overlap zone. These dummy fingers can be insulated from the other electrode fingers or, alternatively, can be electrically connected with the adjacent electrode fingers.
These known dummy fingers have the same width as the other electrode fingers, in other words, about a quarter of the synchronous wave length of the surface wave. Consequently, such fingers have the disadvantage that the reflections of the surface waves at the edges of the fingers are added at the synchronous frequency impairing the amplitude and phase frequency response right in the middle of the transmission band. So as to eliminate these annoying reflections, it has been proposed to provide that the electrode fingers be formed as so-called split-fingers. For this construction, each of the electrode fingers is formed as a double finger but this results in undesired narrow metallized zones which are not only very delicate and difficult to produce but also have increased ohmic resistance and, thus, increase the loss of the electrical energy.